1. Technical Field
The present invention relates to an image processing device configured to compress image data to be used for printing, etc., and in particular an image processing device configured to perform faster printing of images of higher quality with a limited memory size necessary for data storage, etc.
2. Related Art
It is generally known that a page printer such as a laser printer is configured to generate image data of an image to be printed on the basis of a print request, and to perform a printing process in accordance with the image data. So as to efficiently use the capacity of a built-in memory, the page printer usually compresses the generated image data, stores the compressed data in the memory and decompresses the stored data to be used for the printing process.
Various kinds of methods for compressing image data, which use reversible compression and irreversible compression, may be used for compressing the image data as described above. The compression rates of each of the methods are usually traded off against image quality obtained after decompression. If the image data are compressed with a high compression rate, the image quality may usually be degraded in the printing process using the compressed data.
Several methods have been disclosed with respect to such compression of image data. For example, a device configured to perform faster printing of images of higher quality without having to increase a memory size is disclosed in JP-2006-157444. The device of JP-2006-157444 is configured to compress data to be printed on a line-by-line basis, and to further compress the data on a page-by-page basis if the compressed data of a page still has a size larger than a desired size.
Another device configured to reduce processing time without enlarging a circuit scale is disclosed in JP-2007-13266. The device of JP-2007-13266 is configured to compress an image on a divided area-by-area basis by using a selected compression parameter, and to recompress the image by using a compression parameter of a higher compression rate if the compressed image is estimated not to be contained in a compression data storage area.
As the device of JP-2006-157444, however, compresses the data on a line-by-line basis before reaching a uniform compression rate, different compression methods may be applied to lines of different data as a matter of course. In this case, if the compression method is irreversible, reproducibility after decompression may differ among the lines. There is a problem that an image formed on the basis of the compressed data may possibly lose homogeneity, and that the image quality may possibly be degraded thereby.
The device of JP-2007-13266 may need long processing time for processing all the compressed data and recompressing the image from a first divided area by using a new compression parameter if the compressed image is estimated not to be contained in the compression data storage area. In a case where, in particular, no less than three compression parameters of different compression rates are used, there is a problem that the device of JP-2007-13266 may possibly take long processing time.